With the increasing market penetration of portable electronic devices such as telephones, computers, and digital assistants, there has been a corresponding increase in the need for and requirements of batteries to power the devices. In particular, the market demands that the batteries be economical, flexible in application, and rechargeable. Lithium ion batteries have become popular as rechargeable batteries in this market.
Batteries can generally be described as comprising three components: an anode that contains a material that is oxidized (yields electrons) during discharge of the battery (i.e., while it is providing power); a cathode that contains a material that is reduced (accepts electrons) during discharge of the battery; and an electrolyte that provides for transfer of ions between the cathode and anode. During discharge, the anode is the negative pole of the battery and the cathode is the positive pole. Batteries can be more specifically characterized by the specific materials that make up each of these three components. Selection of these components can yield batteries having specific voltage and discharge characteristics that can be optimized for particular applications.
Batteries can also be generally categorized as being “primary,” where the electrochemical reaction is essentially irreversible, so that the battery becomes unusable once discharged; and “secondary,” where the electrochemical reaction is, at least in part, reversible so that the battery can be recharged and used more than once. Secondary batteries are increasingly used in many applications, because of their convenience, reduced cost, and environmental benefits.
There are a variety of secondary battery systems known in the art. Among such systems are lead-acid, nickel-cadmium, nickel-zinc, nickel-iron, silver oxide, nickel metal hydride, rechargeable zinc-manganese dioxide, zinc-bromide, and lithium and lithium ion batteries. Systems containing lithium and sodium afford many potential benefits, because these metals are light in weight, while possessing high standard potentials. For a variety of reasons, lithium batteries are, in particular, commercially attractive because of their high energy density, higher cell voltages and long shelf-life.
Lithium batteries are prepared from one or more electrochemical cells containing electrochemically active (electroactive) materials. Among such batteries are those having metallic lithium anodes and metal chalcogenide (oxide cathodes, typically referred to as “lithium metal” batteries. The electrolyte typically comprises a salt of lithium dissolved in one or more solvents, typically non aqueous aprotic organic solvents. Other electrolytes are solid electrolytes (typically polymeric matrixes) that contain an ionic conductive medium (typically a lithium containing salt in organic solvents) in combination with a polymer that itself may be ionically conductive but electrically insulating.
Cells having metallic lithium anode and metal chalcogenide cathode are charged in an initial condition. During discharge, lithium metal yields electrons to an electrical circuit at the anode. Positively charged ions are created that pass through the electrolyte to the electrochemically active (electroactive) material of the cathode. The electrons from the anode pass through the external circuit, powering the device, and return to the cathode.
Another lithium battery uses an “insertion anode” rather than lithium metal, and is typically referred to as a “lithium ion” battery. Insertion or intercalation electrodes contain materials having a lattice structure into which an ion can be inserted and subsequently extracted. Rather than chemically altering the intercalation material, the ions slightly expand the internal lattice lengths of the compound without extensive bond breakage or atomic reorganization. Insertion anodes contain, for example, lithium metal chalcogenide, lithium metal oxide or carbon materials such as coke and graphite. These negative electrodes are used with lithium-containing insertion cathodes. In their initial condition, the cells are not charged, since the anode does not contain a source of cations. Thus, before use, such cells must be charged in order to transfer cations (lithium) to the anode from the cathode. During discharge the lithium is then transferred from the anode back to the cathode. During subsequent recharge, the lithium is again transferred back to the anode where it reinserts. This back-and-forth transport of lithium ions (Li+) between the anode and cathode during charge and discharge cycles has led to these cells as being called “rocking chair” batteries.
Electrochemically active materials for the cathodes of such batteries must be economical, easy to manufacture, and capable of reversibly being oxidized and reduced during the charge-discharge cycles. Commonly used active materials include lithium transition metal oxides such as LiCoO2, LiNiO2, and LiMn2O4. These materials all have drawbacks, including expense, difficulty of manufacture, and loss of battery capacity on cycling. There is a constant search in the industry for new active materials with improved properties, and for methods for synthesizing the active materials.
Gaubicher and coworkers (see, for example, J. Electrochem. Soc. 146, 4375 (1999), Electrochem. Solid State Lett. 3, 460, (2000)) have described electrochemical and chemical lithium insertion into chemical species such as VOXO4, where X can be sulfur, phosphorus or arsenic. Lithium is inserted electrochemically by providing the VOXO4 material in an electrochemical cell opposite a lithium counter-electrode. Upon discharge, the lithium ions are inserted into the VOXO4, with concomitant reduction of the vanadium species. In the chemical insertion reaction, a solution of lithium iodide is reacted with VOPO4, producing a material stoichiometrically equivalent to LixVOPO4. Lii and coworkers (see, for example, J. Chinese Chem. Soc. 39, 569 (1992), Zeit. Kristall. 197, 67 (1991)) have reported on the properties and synthesis of sodium vanadyl phosphate, prepared using both solid state and hydrothermal methods.
It would be desirable to provide improved methods for synthesis of vanadyl materials. It would further be desirable to develop reductive methods that produce active materials intimately mixed with conductive carbon material for direct use in fabricating electrodes for electrochemical cells. Further, it would be desirable to develop new vanadyl materials for use as electrochemical active materials and rechargeable batteries.